This invention relates to a switched capacitor filter for filtering a filter input signal to produce a filter output signal by carrying out sampling operation.
As will later be described with reference to one of several figures of the accompanying drawing, a conventional switched capacitor filter of the type described comprises a capacitor, a sampling circuit for sampling the filter input signal at a sampling rate to produce a switched signal at the sampling rate, and an integrating circuit for integrating the switched signal to produce a filter output signal. The integrating circuit has an operational amplifier and an integrating capacitor inserted in a feedback path of the operational amplifier. The switched capacitor filter is specified by a transfer function dependent on a capacitor ratio of the integrating capacitor relative to a unit or a minimum capacitor.
A switched capacitor filter of the type described is proposed in a technical paper, IEEE International Symposium on Circuits and Systems, April 1980, pp. 309-312, contributed by Titkwan Hui and David J. Allstot, under the title of "MOS SWITCHED CAPACITOR HIGHPASS/NOTCH LADDER FILTERS".
It is pointed out in the Hui et al paper that a very large capacitor ratio is required in such a switched capacitor filter when the sampling rate becomes high as compared with an audio baseband frequency, such as 50 Hz, 60 Hz, or the like. This means that a very wide area should be occupied by the integrating capacitor on large scale integration. In order to minimize the occupied area, the unit capacitance must be chosen as low as possible. As will later be described with reference to another figure of the accompanying drawing, the Hui et al paper teaches use of a capacitive divider so as to reduce the capacitor ratio.
However, such a capacitive divider inevitably accompanies a stray capacitance, as will later be discussed. Inasmuch as the stray capacitance adversely affects a filter characteristic, it is necessary to reduce the stray capacitance by augmenting the unit capacitance. In this respect, the integrating capacitor should have a large capacitance. A total capacitance inevitably becomes large. Therefore, an area occupied by the capacitors relatively becomes wide on the large scale integration of the switched capacitor filter. The semiconductor chip must have a large size.
In addition, electric power consumption is increased with the switched capacitor filter because the operational amplifier should drive a large capacitance.